This invention relates to high-efficiency, on-axis, multilevel, diffractive optical elements. The high efficiency of these elements allows planar or spherical elements to be diffractively converted to generalized aspheres, and dispersive materials can be diffractively compensated to behave as achromatic materials over broad wavebands. The technique cf this disclosure allows ready implementation of this mixed reflective, refractive and diffractive optics in real systems.
The ability to produce arbitrary phase profiles allows for an additional degree of freedom in designing optical systems. Many optical systems now incorporate aspheric refractive surfaces to produce such phase profiles. System design is restricted by constraints imposed by factors such as cost, size, and allowable asphericity. Diffractive elements are potentially as versatile and useful as aspheric surfaces and are less expensive, and not as subject to asphericity constraints. Another objective in designing optical systems is to minimize chromatic aberrations. Refractive optical materials are chromatically dispersive. Conventionally, the approach to minimizing chromatic aberrations is to balance the dispersive effects of two different refractive materials. Diffractive surfaces are also wavelength dispersive. It is therefore possible to take a dispersive refractive element, and by placing a diffractive profile on one of its surfaces, produce an element that balances the chromatic effects of the refractive element against the chromatic effects of the diffractive surface. Computer generated diffractive elements have been proposed for numerous applications such as chromatic correction, aberration compensated scanners, and hiqh numerical aperture lenses. A major obstacle to implementing on-axis diffractive elements in actual systems is the, up to now, low diffraction efficiency (&lt;50%).
Theoretically, on-axis diffractive phase elements can achieve 100% diffraction efficiency. To achieve this efficiency, however, a continuous phase profile is necessary (See, Miyamotc, K., 1961, JOSA 51, 17 and Lesem, L., Hirsch, P., Jordan, J., 1969, IBM J. Res. Dev 13, 150.) The technology for producing hiqh-quality, hiqh-efficiency, continuous phase profiles does not exist. It has been suggested to quantize the continuous phase profile into discrete phase levels as an approximation to the continuous phase profile. (Goodman, J., Silvestri, A., 1970, IBM J. Res. Dev. 14, 478.) It is known to make such structures using thin-film deposition techniques and material cutting technology (See, U K. Patent Application No. 8327520 entitled "Bifocal Contact Lenses Having Diffractive Power".) L. d'Auria et al. in "Photolithographic Fabrication of Thin Film Lenses", OPTICS COMMUNICATIONS, Volume 5, Number 4, July, 1972 discloses a multilevel structure involving successive maskings and etchings of a silicon dioxide layer. Each mask gives only one additional level in the structure and is therefore inefficient. The invention disclosed herein is a method for accurately and reliably making multilevel diffractive surfaces with diffraction efficiencies that can be as hiqh as 99%.